1. Field of the Invention
The present invention relates to a method and apparatus for the handling of overburden removed from the top of a seam of mineable mineral, e.g., coal located near the surface of the earth during strip mining operations.
2. Description of the Prior Art
Mining of minerals, e.g., coal, in the United States and foreign countries takes a number of forms, including strip mining. Strip mining is a process whereby a seam of mineable minerals close to the surface of the earth is removed by first stripping away the unwanted earthen materials, i.e., dirt, called the overburden, from above the seam and then mining the desired minerals contained in the seam.
This removal of the overburden often requires the removal and relocation of massive amounts of earthen dirt from above the mineral seam. The methods and apparatus for doing this removal and relocation determine the economic feasibility of strip mining the mineral seam, because it is only economical to remove so much dirt in so much time to uncover a mineral seam containing a given quantity of desired mineral. The costs in terms of labor, capital investment in equipment, lease costs and energy consumed must be decreased to a minimum in order to fully exploit seams of mineable minerals close to the earth's surface. The cheaper it is to remove the overburden from the mineral seam per unit volume of overburden, the deeper the mean depth of a seam can be from the surface while maintaining the same profit margin. Therefore, the resources available through strip mining can be increased by decreasing overburden removal expenses.
Typically in a strip mining operation the overburden is removed from a generally rectangular area above a seam, e.g., a coal seam. The coal is then removed leaving a hole in the ground, referred to in the art as a pit. The pit can be of varying dimensions and will have walls of varying heights depending on the topography and the depth of the seam below the earth's surface at the place the pit is formed. In the prior art it was most desirable for a strip mining operator to form the first pit where the overburden depth was a minimum in order to be able to commence actual production of the coal seam with a minimum overburden removal time. This allowed the prompt generation of income to sustain further operation of the strip mine. Therefore, prior art strip mines typically contained a first pit having front and back walls running along a topographical line of generally the least amount of overburden cover.
In the art of strip mining it is the usual procedure to remove overburden from an area adjacent to the first pit and place this removed overburden in the first pit, thus forming a second pit having a front wall of unremoved overburden and a back wall of the removed overburden which was deposited in the first pit. This removed overburden placed in the previously mined pit is called a spoil pile.
Two problems exist in the prior art. Both of these are related to the increasing scrutiny of federal and state governments of the condition in which strip mining operators leave the land after the mining is completed.
Because of the necessity to start a first pit along a line where the overburden is thinnest, the second pit will be deeper than the first. This assumes the coal seam runs generally horizontally in the area being mined and the topography of the land in the area being mined is not perfectly flat. The next pit after the second pit, the overburden from which is placed in the second pit, and succeeding pits will continue to increase in depth until the pits are advanced across the area being mined as far as possible or desirable. When this is done the last and deepest pit is left unfilled. Previous practice was to leave this unfilled or partially filled, marring the landscape. Federal and state regulations concerning regrading of strip mines now require refilling of this last pit to reconform the whole mined area to the general topography existing before the mining occurred.
A second and related problem stems from the prior art methods of transporting and depositing the overburden on the spoil pile being extended across the previously mined pit. Typically the spoil pile was formed of a plurality of discrete piles. The terrain in the whole mined area after strip mining, using prior art methods, thus contained numerous peaks which had to be leveled at great expense in time, labor and energy use, in order to regrade the whole mined area to meet regulatory requirements.
Thus, an apparatus and method was found to be needed which would enable fast and efficient removal of overburden with the pits being formed substantially perpendicular to the topographical line of minimum overburden depth. This would substantially reduce the first problem, since the final pit would not be the great hole resulting where the final pit runs generally along the topographical line of maximum depth of overburden removed from above the coal. Rather the final pit would vary in depth, having a shallow end and a deep end and would thus require less dirt to be relocated to fill it.
It was also found that a quicker and more efficient apparatus and method were needed to transport and deposit the overburden onto the spoil pile while at the same time being capable of substantially evenly distributing this dirt so that regrading would be substantially unnecessary.
Prior art conveyor apparatus are shown in the patents to Mercier, U.S. Pat. No. 2,642,984, Waterman, Jr., U.S. Pat. No. 1,833,192, and Coppock, U.S. Pat. No. 1,821,881. These fail to meet the needs of the strip mining problems noted above because they lack full articulation in three dimensions.
Prior art methods of removing the overburden from the coal layer and depositing it on the continuation of the spoil pile include shovel and drag line operations. The shovels and drag lines are necessarily of extremely large size, running upward to 70 tons. The booms on these machines must be of great length in order to be able to transport the overburden from where it is being removed to the spoil pile without the necessity of moving the base of the shovel or drag line. Such an operation is necessarily slow and consumes a great deal of energy required to lift the drag line or shovel high enough to dump the material on top of the extension of the spoil pile and to swing the boom from the overburden bank to the spoil pile and back. In such operations, also, the spoil pile is built up in a haphazard manner forming a series of peaks which require further grading after the mining operation is completed in order to level off the area in compliance with numerous federal and state regulations governing strip mining operations.
Further prior art methods of transporting the overburden to the spoil pile require the use of shovels or drag lines, or smaller excavation and lifting devices, such as 5 to 15 cubic yard loaders, scrapers, or excavator loaders. Each of these can be used to lift the overburden into hoppers on the backs of large wheeled or tracked vehicles. These vehicles are driven onto the spoil pile, where the overburden is dumped from the hoppers onto the spoil pile.
It has been found that prior art methods of and apparatus for transporting overburden from the overburden bank to the spoil pile are slow and consume a great deal of energy, and suffer from the infirmity noted above of distributing of the overburden on the spoil pile in a haphazard and nonuniform manner.
Economic feasibility in removing the overburden can be measured in the form of a stripping ratio of overburden to mined material. For example, a ratio of 14 cubic ft. of overburden to 1 cubic ft. of coal may be the acceptable number in some geographic regions. Present methods of overburden transfer limit production and increase costs to the point of reducing the mineable reserves of coal produceable by strip mining in the United States to thick seams or shallow overburden.
Methods of strip mining of coal vary throughout the country, depending on many variables such as overburden materials and depth, the lay of the coal seam, thickness of the coal seam, depth of the coal seam, and both financial and functional operational choices.
In each of the above cases total production time, including overburden stripping, is limited by materials, transport time, availability of the machines, interdependence of the machines, and reliability. A significant portion of the cycle time in removing overburden is used in transferring the removed overburden from the overburden bank to the spoil pile. Any reduction in this time has been found to result in a direct increase in productivity. As already noted, draglines and stripping shovels have historically left spoils in a configuration that requires extensive and expensive reworking, such as, refilling and grading. Thus, for example, typical dragline and stripping shovel spoil piles could contain approximately 9680 loose cubic yards (1 cy) per acre of material which must be moved at a cost of approximately $1000.00 per acre in order to regrade the mined area. This cost and material handling is in addition to that required for removing the overburden to mine the coal.
A further consideration is the energy expended in the prior art types of apparatus and methods of removing and transporting the overburden. Shovels and drag-lines expend a large part of their total energy in lifting the overburden load to the height of the spoil pile and swinging the load over to the spoil pile then swinging back to the overburden removal site. Loaders expend considerable energy in transiting between the overburden removal site and the top of the spoil pile.